The invention relates to a mobile radio telephone station comprising traffic and identification encryption means, a protection system for at least one authentication number, and storage means for storing said authentication number.
The invention likewise relates to a method of protecting an authentication number contained in a mobile radio telephone station of the type mentioned above.
Such a protection finds interesting applications notably in the field of mobile radio telephony of the AMPS type (or TACS and ETACS) and also of the CDMA type. Important numbers in this field are, notably, the identification numbers (ESN and MIN) of the telephony devices which can be connected to the AMPS network, numbers which correspond to the serial number of the telephony device and to the latter's identification number, respectively. These numbers are the prey of pirates who try to appropriate a code that does not belong to them, so that the communications are charged to another person than themselves.
In order to protect oneself against this type of dishonest manoeuvres, a protection system is known which is described in the opening paragraph of U.S. Pat. No. 5,392,356. According to this known system, the identification code(s) is (are) encrypted in the mobile station before being written in the EEPROM of the mobile station, after which the codes are then decrypted and transmitted the moment the communication is sent. Such a trick prevents the identification codes from being extracted from the EEPROM by hardware means and thus from being acquired in a simple manner.
However, identification codes such as the MIN and ESN can always be acquired by an interception of a communication between the mobile station and the base station, which makes aforesaid fraud still possible.
For maintaining the integrity of the mobile stations and precluding their being pirated, one has gradually been led to introduce more sophisticated authentication procedures as the pirating of mobile stations increased. From now on, in many networks an authentication number is assigned to each mobile station; this is the authentication key or A-key, and a second authentication number, the SSD (Shared Secret Data) is computed by encryption based on the A-key.